Relatively compact electrical testers which are normally hand held in use and sufficiently compact to be carried in the user's pocket, which provide a very simple signal output, without recourse to meters, digital read outs or the like, have long been known. However, past devices of which I am aware have not been fully satisfactory and have suffered from one or more disadvantages. For example, certain of these prior devices have been incapable of testing the full range of commonly desired electrical characteristics, including circuit continuity, voltage and polarity (e.g., DC voltage polarity and diode polarity). Oftentimes, those prior devices capable of both voltage and continuity tests have utilized separate circuits for each, increasing circuit complexity and cost. Frequently, prior devices have resorted to plural and often different indicator devices for providing the results of different tests and/or have been usable over only a relatively narrow voltage and/or resistance range. Indicators on prior test devices have frequently been light sources which vary substantially in output over the usable range of the instrument so as to be misleading or difficult to read under varying ambient light conditions. Complexity of circuitry or mechanical construction has also been a problem.
Further, little has been done, insofar as I am aware, to avoid damage to the tester during use, particularly instrument damage due to operator inattention or misuse. Such instrument damage is particularly a problem with test instruments of the type capable of testing parameters of both "live" circuits and non-energized circuits or components, i.e., capable of testing both voltage and continuity.
Thus, the objects and purposes of this invention include the provision of:
1. An instrument for testing electrical continuity, voltage and polarity wherein, within the operating voltage range of the instrument, the instrument is protected against inadvertent connection across a voltage source while in its continuity mode.
2. An instrument, as aforesaid, capable of testing both AC and DC voltages over a wide voltage range, capable of indicating DC voltage polarity, capable of continuity testing over a wide resistance range and capable of checking polarlity of a polarized nonlinear device such as a diode, and which provides a "GO-NO GO" indication for each such test.
3. An instrument, as aforesaid, which is capable of providing an indication for all of these different types of tests with a single visible indicator of light-emitting type, and the instrument includes a single circuit loop containing the indicator light source and an electrical supply which for all types of tests provides in normal use the sole current source for the indicator light.
4. An instrument, as aforesaid, in which the indicator is a low current drain, substantially constant light output illumination source.
5. An instrument, as aforesaid, which includes a manual switch actuable to alternatively select voltage and continuity modes of the instrument wherein the light source is protected against damage from connection of the instrument to a voltage source with such manual switch in continuity position.
Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.